Towing operations on ocean vessels are normally carried out by large towing winches having drums around which the towing cable is wound. Mooring winches of generally similar construction serve to anchor semi-submersible drilling rigs in a fixed position over a well that is being drilled. In both cases, the winch is rotated by a drive system that is usually powered by an electric motor connected to the winch shaft by a clutch mechanism and a gear train. Large winches of this type are equipped with friction brakes that apply a braking force to the winch drum when applied.
The winches can be single units or multiple unit systems. The weight of the cable and the heavy sea conditions that are often encountered exert extremely heavy loads on the winch. To exemplify the size of the equipment necessary to handle the loads, it is not uncommon for the drums to weigh several hundred thousand pounds and for thousands of feet of cable or wire rope to be wound on the drum. The cable or wire rope can be several inches in diameter. Drive motors capable of producing hundreds of thousands of pounds of line pull are used. The dual band brakes that are commonly used are capable of withstanding several million pounds of force.
Both towing winches and mooring winches are ordinarily provided with a ratchet mechanism which positively locks the winch drum against rotation in a direction to pay out more cable. A typical arrangement includes a dog or pawl which acts against a ratchet wheel on the drum. In the case of a towing winch, the pawl eliminates the possibility of brake slippage at maximum loads and thus increases the resistance of the winch to shock loading. Also, the pawl permits smaller friction brakes to be used and prevents the cable from becoming lost in the event of loss of winch power. In the case of a mooring winch, the pawl eliminates movement except that caused by anchor slip. Due to the locking function of the pawl, the winch motive power can be diverted to the drilling equipment without relying wholly on the friction brakes to maintain the winch position. If there is an equipment malfunction, the locking pawl assures that the position of the drill rig is properly maintained.
The positioning of the pawl is controlled either manually or by means of an actuator such as a pneumatically or hydraulically powered cylinder. The pawl is engaged by rotating it into contact with the ratchet wheel on the winch drum, and the drum is slowly rotated until the tip of the pawl is firmly engaged in one of the notches in the ratchet wheel. So long as there is a force applied to the drum in the direction of the pawl engagement, the pawl is held in the engaged position to lock the winch against rotation. Rotation of the drum in the opposite direction is permitted as the pawl rides along the ratchet wheel into and out of excessive notches. Release of the pawl under normal conditions involves rotation of the winch drum away from the pawl and release of the force which holds the pawl against the ratchet wheel. Engagement and disengagement of the pawl is facilitated by minimizing the length of the pawl without diminishing its ability to withstand the compression loads that are applied to it by the winch drum.
If an emergency should arise such as a blow out of the well, it is sometimes necessary to move the drill rig from its position over the well in the absence of electrical power. Before the drill rig can be moved in an emergency situation, both the winch brakes and the pawl must be released without electrical power and with the winch drum under heavy load. The winch cannot be rotated due to the loss of its motive power, and the heavy load exerted on the pawl by the weight of the cable and the effect of sea forces makes it difficult if not impossible to disengage the pawl under these conditions.
The devices that have been proposed in the past for effecting emergency release of the pawl have not been satisfactory. Due to the absence of electric power, only air or hydraulic power is available. The need to develop extremely large release forces has resulted in attempts to construct large actuators that operate long levers. Under conditions of heavy winch loading, the force required to disengage the pawl exceeds the force that can be applied by a pneumatic or hydraulic device acting alone. Even if the necessary force can be produced, large actuators are unacceptable because their unduly large size cannot be accommodated in the limited space that is available. Also, the expense is excessive if not prohibitive. The emergency operating source, air or hydraulic fluid, must be very large in order to provide the required force, and this further detracts from the practicality of previously proposed emergency release devices. Another drawback is that the brakes must be released by a separate independent operation.
It has also been proposed that a cutting torch be used to physically cut the pawl or cable in an emergency situation. This type of operation requires the presence of a worker at the winch and can result in possibly serious personal injury. Furthermore, physically cutting the pawl or cable permanently damages the equipment and leads to delay that can be too great to meet the time constraints that are usually involved in releasing the pawl under emergency conditions. Again, the brakes must be released independently.